The present invention relates to a method or process of working up an overhead product of an extractive distillation of a hydrocarbon mixture. It also relates to an apparatus for performing that process.
A method or process of working up an overhead product of an extractive distillation of a hydrocarbon mixture is known, in which N-substituted morpholines, whose substituents do not have more than seven carbon atoms are used as selective solvents. The lower boiling components of the hydrocarbon mixture serving as starting product are drawn off as an overhead product from the head of the extractive distillation column. After that the overhead product is distilled for recovery of the residual solvent present in it and the sump product coming down with a certain amount of solvent is taken from the overhead product distillation column and is separated into a light phase and a heavy phase. The heavy phase is subsequently fed back into the extractive distillation column and the light phase is fed back to the overhead product distillation column.
The above-described extractive distillation process has been known for several years and used for separating hydrocarbon mixtures of differing compositions, for example for separation of aromates and nonaromates or for separation of olefins and/or diolefins and paraffins. In large scale industrial processing this process has proven especially useful for producing high-purity aromates using N-formyl morpholine as a selective solvent. In performing this process the sump product drawn from the extractive distillation column is normally fed to a subsequent decanter column, in which the hydrocarbons contained in it are separated as a distillative extract from the solvent. The solvent is then drawn off from the sump of the decanter column and returned to the extractive distillation column. The feed and/or return of the solvent occurs normally to the head of the extractive distillation column. Because of that the production of an overhead product with a certain amount of solvent can not be avoided in practice, the solvent content in the overhead product amounting to about 2% by weight. Because of economic reasons and because as pure as possible an overhead product must be obtained it is absolutely necessary to recover this solvent component again from the overhead product.
This would certainly be possible, if one drives the extractive distillation column with a suitably high overhead product reflux. In contrast to the normal distillation however in the extractive distillation a reflux of this type is inapplicable and thus must be avoided because:
1. An overhead product reflux leads to a dilution of the solvent and thus to a selective reduction, whereby the desired material separation is made unnecessarily difficult.
2. Highly selective solvent -- and here the above-named N-substituted Morpholines -- have only a limited dissolving power for the low boiling hydrocarbon component. The overhead product reflux can thus lead to the formation of two fluid phases with different densities in the upper plates of the extraction column, which makes impossible a problem-free operation of the extractive distillation column.
This possibility for recovery of the solvent component from the overhead product is obviously eliminated and a separate recovery of the solvent from the overhead product must occur. This can be performed of course by a simple distillation of the overhead product in such a way that the overhead product is withdrawn as a top product with a solvent content of &lt;10 ppm from the distillation column, while the solvent concentrated to a purity of almost 100% is taken from the sump of this column and is fed back to the extractive distillation column. This operation, in which as complete as possible a separation of overhead product and solvent is desired, requires however a highly expensive apparatus(distillation column with a high plate number) and a high energy consumption.
In German Patent Application 34 09 030 a distillative separation of the solvent from the hydrocarbon of the overhead product which is only incompletely undertaken is described. This occurs by drawing a sump product from the overhead product distillation column, which still has a a portion of solvent therein. Subsequently this sump product is fed after a suitable cooling into a separating vessel, in which it separates into a light phase and a heavy phase. The heavy phase comprises a solvent and the hydrocarbons of the extract, which are present in the overhead product as impurities. Because of their composition they can be fed back into the extractive distillation column, while the light phase, which contains the usual components of the sump product, is fed back into the overhead product distillation column.
In performing the above-described process it has been shown in some cases that the effectiveness of the separating vessel is not satisfactory. This was the case particularly when the components of the heavy phase were put in the sump product drawn from the overhead product distillation column in the form of very fine drops, whose sedimentation speed was less than the rising speed of the components of the light phase. In this case the components of the heavy phase were fed back to an undesirable extent into the overhead product distillation column and thus the separation efficiency of this column was poor.